It is most likely a 12 volt system. I have yet to run across a skoolie (as far as i know) that was 24 volts. I don' drive a truck for a living, but I run across quite a few fire trucks and other rescue equipment on big truck chassis, and they've all been 12 volt as well.
24 just isn't that common in north america......outside the military. The US govt. is one place i've seen a lot of 24 volt vehicles. Someone told me our military vehicles are set up the same as other vehicles in NATO or something like that...
If you look at the battery, it should tell you how many volts it is. If there is more than one battery, you would have to see if they were wired in paralell or series. Perhaps an easier method would be to remove a light bulb from anywhere in the bus and see what the voltage rating is. That should be clearly marked on the base of the bulb. You could always plug in a standard 12 volt light bulb to the bus and see what happens. If it keeps working for more than a nano-second, then the bus is a 12 volt system. A volt meter would also tell you the votage. Remember it won't be exactly 12 or 24 volts as these numbers are nominal. Perhaps the easiert method of all would be to ask the seller.
What kind of bus are you looking at?
Quis custodiet ipsos custodes (who will watch the watchmen?)